One Universe at a Time

Emergence Of Gravity

As dark matter continues to vex astronomers, new solutions to the dark matter question are proposed. Most focus on pinning down the form of dark matter, while others propose modifying gravity to account for the effect. But a third proposal is simply to remove gravity from the equation. What if the effects of gravity aren’t due to some fundamental force, but are rather an emergent effect due to other fundamental interactions? A new paper proposes just that, and if correct it could also explain the effects of dark matter.

The idea of emergent gravity isn’t entirely new. The most popular variation was proposed in 2010, where Erik Verlinde argued that gravity is not a fundamental force, but rather an effect that arises from the entropy of the Universe. Entropy is a property of thermodynamics. It’s often described as the unusable part of a system (or the waste heat if you will) and while that’s sometimes a useful description, a better description involves the amount of information contained within a system. An ordered system (say, marbles evenly spaced in a grid) is easy to describe because the objects have simple relations to each other. On the other hand, a disordered system (marbles randomly scattered) take more information to describe, because there isn’t a simple pattern to them. Basically, the more information it takes to describe a system, the more entropy it has.

Verlinde’s model uses this connection between thermodynamics (heat, energy, and forces) and information through a mathematical method known as the holographic principle. Since the information contained within a region of space depends upon the arrangement of objects within that region, moving the objects can change the entropy within the region. Verlinde demonstrated that this produces an entropic force that acts like gravity. From the basic idea of information entropy, one can derive Einstein’s equations of general relativity exactly.

Entropic gravity is an interesting idea, and it would explain why gravity is so difficult to bring into the fold of quantum physics, but it’s not without its problems. For one, since entropic gravity predicts exactly the same gravitational behavior as general relativity, there’s no experimental way to distinguish it as a better theory. There are also theoretical problems with the model. For example, if you try to describe a gravitationally closed system of masses within the model it only matches experiment if you place weird constraints on the entropy of the system.

But despite its problems the idea is at least worth exploring, and this latest work adds a new twist by describing the effects of dark matter. In the original formulation, the model focused on standard gravity. Specifically, it excluded dark energy. This new paper notes that since the dark energy of a region of space requires additional information to describe, including it in the model changes the entropy of a region of space. The paper then goes on to show how this additional information creates an additional entropic force. One that might account for the effects of dark matter similar to other modified gravity models such as Modified Newtonian Dynamics (MoND). Thus gravity, dark matter, and dark energy might all be connected through entropy.

While this seems like an elegant solution to several cosmological problems, there are plenty of reasons to be skeptical. For one, this new variation of emergent gravity still has the same theoretical difficulties of the original. Then there’s the fact that modified gravity models fail to explain large scale effects such as the clustering of galaxies, which regular gravity and dark matter explains very well. This new work is still more of an idea and less of a robust theory.

But even if the model doesn’t work out in the end, it demonstrates how thermodynamics and gravity are deeply connected in ways that aren’t obvious at first glance.

Comments

I have a suspicion that Verlinde is onto something huge. As you say, a lot remains to be sorted out, but if you can explain both dark energy and matter and as a sideline the gravitational behaviour of galaxies and galaxy clusters by a parameter free (almost) theoretical argument based on string theory, I call it a good days work.

I am no expert….but I have a theory…the Ligo Scientic corporation states that the cause of gravitational wave is rotation of spherically non-symmetric mass about a fixed axis . for example, a perfectly spherical star spinning about its axis would not produce a gravitational wave but a star with a lump on its surface would emit gravitational waves, now, relating this to entropic energy : a perfectly spherical star would not have a change in entropy when it spins, as the physical appearence of the star taken at any two time instants is exactly the same. thus, there is no entropic force acting & hence there is no gravitational wave. But when a star with a lump rotates, its physical appearence would be same only if the time interval is an integral multiple of its period of rotation. i.e, if at one instant the lump is on right side then after a few instants the lump would be on left side ( for time interval = time period/2), so that means there is no spherical symmetry when it rotates or even if its stationary. when this star rotates, the entropy of the system is continuously changing because in each instant the position of the lump changes. thus, the entropic energy associated with the star will continually change. if gravity is an emergent force due to change in entropy, then that means, an object that is not spherically symmetrical when rotates will continually radiate gravitational waves due to the continuous change in entropy associated with it. Damn ! that indeed would explain GW. Lol !

Another aspect of Verlinde’s latest paper is that, in addition to entropic and holographic considerations, he treats Spacetime (actually static ‘space’) like an elastic solid, and addresses the stress-strain relationship using the usual linear approximation. Using similar thermodynamic and holographic considerations, and treating Spacetime as a dynamic elastic solid, one can calculate that the total energy density is the Critical Friedmann value that makes space ‘flat’, as observed. One can also predict that the energy density of Dark Energy is 2/3 of the total – again, in good agreement with observations.

Describing gravity, dark matter, and dark energy in terms of entropy seems to me like relating four dependent variables without knowing what is independent. This would explain the bizarre constraints required for it to work in a closed system. Elegant as it is, I haven’t bought into this approach yet.